The invention relates to a method for improved manufacture of strip-shaped and/or point-shaped electrically conducting contacts on a semiconductor element such as a solar cell. The invention also makes reference to a method for manufacturing a composite of semiconductor elements, such as a solar module.
When manufacturing electronic components, fine electrically conducting structures are applied primarily by means of physical and chemical gas-phase deposition, galvanic processes using masks or possible with additional laser support. These techniques permit manufacture of very fine structures, even though, for economic reasons, they are hardly suitable for cost-effective mass production.
With manufacture of solar cells, in regard to the side that is faced toward the irradiation, a requirement exists to apply electrically conducting structures that are as fine as possible, which ensure good electrical conductivity as well as good electrical contact to the solar cell. This is required because the surface facing toward the irradiation is to be in shadow as little as possible.
However, to make possible good electrical conductivity with high current dissipation, the conductor in question must have a large cross section.
To meet these requirements, according to the prior art, contacts are often applied via screen printing, which can be amplified by galvanic processes.
According to WO-A-91/24934, an electrically conducting paste can be applied onto a carrier, which is polymerized and stabilized by irradiation with UV light.
To form a structure on a plasma screen, according to U.S. Pat. No. 6,312,864 it is known to apply a substance with a binder to be decomposed by heat, which then can is cured by thermal action. U.S. Pat. No. 6,433,620 proposes curing of a substance on a carrier by thermal treatment.
A paste is known from JP-A-63268773 that is free of solvents and contains a precious metal powder, fritted glass, metal oxide and a binder.
When the pasty substances are applied to form electrically conducting contacts, pressure-application procedures can be used. However, fundamentally these display a disadvantage in that if the line is narrow, it is not possible for the layer to have great thickness. This results in the disadvantage that wider lines, or a greater number of them, are required to attain the desired low contact resistance values.
From WO-A-2005/088730, a procedure is known for forming a linear and/or a point-shaped structure on a solar cell, in which an electrically conducting pasty substance adhering on the carrier is applied which contains a solvent. To avoid a dissolution of the strip-shaped material after application, and to have the strips pull together in width terms after application, it is proposed that following application of the pasty material, a medium is applied to it that contains polar molecules and that the solvent is extracted at least in part.
The medium containing the polar molecules is especially a surfactant medium in the form of a liquid or a foam.
To avoid solar cells having a flat rear contact that can consist of aluminum, being bent through during the manufacturing process, according to DE-B-10 2005 026 176 it is proposed that after the flat rear contact is applied, the solar cell be heated to a temperature above 567° C. then be cooled to below the manufacturing ambient temperature of the solar cell. Preferred temperature ranges lie between 0° C. and −40° C.
Proper solar cells usually are interconnected to modules in which the solar cells are embedded in plastic layers, preferably made of ethylene vinyl acetate (EVA). On its front side, the module preferably is covered by a pane of glass or some other transparent plate, and on the rear side by a plastic composite foil, for example.
Moisture penetrating into the module, high temperatures and UV irradiation can result in acetic acid forming when EVA is used, which can form acetates with the contacts and the metals present in the glass, thus causing the contacts to corrode.
In DE-A-10 2006 005 026, a procedure is described for manufacturing flat transparent metal oxide surfaces. For this, an electrically conducting metal oxide and a dispersing agent in the form of a layer are applied onto a substrate, and then sintered by microwave irradiation. Between the application and the sintering, a drying step may be carried out.